BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to a rotor sensor target for magnetic bearings.
2. Description of the Related Art
[0002] An active magnetic bearing which may be a radial bearing or an axial (or thrust)
bearing comprises a rotor, a stator fitted with electromagnet windings, at least one
sensor for sensing the radial or axial position of the rotor and servo-control circuits
for maintaining the rotor in equilibrium without contact with the stator, the currents
carried by the electromagnets of the stator being servo-controlled on the basis of
signals delivered by the at least one sensor.
[0003] Figure 9 schematically shows an example of a known radial magnetic bearing 200 comprising
a stator with a stack of ferromagnetic laminations 211 and electromagnet windings
212 and a rotor fitted with a second stack of laminations 213 mounted on a shaft 220.
A radial position detector 201 of the inductive type comprises a stator with a stack
of ferromagnetic laminations 231 and electromagnet windings 232 and a rotor fitted
with a second stack of laminations 233 mounted on the shaft 220. An axial position
detector 202 of the inductive type comprises a stator with stacks of ferromagnetic
laminations 251, 251' and electromagnet windings 252, 252' and a rotor fitted with
second stacks of laminations 253, 253' mounted on the shaft 220. The servo-control
circuits are not illustrated in figure 9. Active magnetic bearings may be configured
in various manners. In particular, as disclosed in
US patent 6849979 B2, radial and/or axial position sensors may be combined and/or integrated in a bearing.
Moreover the ferromagnetic laminations may also be replaced by solid parts of a magnetic
material.
[0004] When the position sensors are of the inductive type, the rings of rotor laminations
such as the stacks of ferromagnetic laminations 233, 253 and 253' in Figure 9 and
their support are called a rotor sensor target.
[0005] Figure 10 illustrates an example of a known rotor sensor target. Target materials
230, 240 are mounted on a shaft 220 which is typically made of carbon steel and are
made integral thereto.
[0006] Generally speaking to make an axial or an axial-radial sensor on the rotor of a magnetic
bearing system, two materials are needed, i.e. a first non-magnetic material and a
second magnetic material, which may be made of laminations or of a solid part. The
axial displacement of the rotor is measured at the border between the magnetic and
the non-magnetic materials. These materials need to have sufficient strength to withstand
high speeds and different temperatures without losing contact to the main shaft or
breaking.
[0007] In the known embodiment illustrated in figure 10, Inconel 718 may be used as a non-magnetic
material constituting a main target component 240 in combination with laminations
230 of magnetic material. The main target component made of non-magnetic material
may comprise a ring 241 having a height H1 which is mounted on the shaft 220 of carbon
steel having a height H2. The laminations 230 of ferromagnetic material are mounted
on the ring 241 and are maintained between an additional ring 242 of non-magnetic
material and a projection 243 of the ring 241.
[0008] The advantage given by the Inconel as a non-magnetic material is its very high mechanical
resistance together with a coefficient of thermal expansion very close to the steel
coefficient of thermal expansion. The addition of these two properties allows the
sensor to be used in a wide range of temperatures and speeds. However there is a disadvantage
of a very high cost due to the Inconel price.
[0009] As an alternative material to Inconel a design with a high resistance stainless steel
(Z6NiCrTiMoVB25-15-2) or High resistance brass (CuNi2Si) has been used for the non-magnetic
material 241, 242 to reduce the cost compared to Inconel with little restrictions
on temperature and speed but a lower cost. The laminations of magnetic material 230
are usually made of FeSi.
[0010] As shown in Figure 10, with the conventional shape of the ring 241 with a projection
243 and of the additional ring 242 all made of non-magnetic material such as Inconel,
high resistance stainless steel or high resistance brass, the height H1 of the material
under the magnetic iron laminations 230 interposed between the ring 242 and the projection
243 and the addition of the height H2 (i.e. the radius) of the shaft 220 under the
magnetic portion 230 of the sensor target gives a very stiff assembly which may be
detrimental to the magnetic portion made of laminations if some cheaper non-magnetic
materials are used for the ring 241 with projection 243 and the ring 242.
SUMMARY OF THE INVENTION
[0011] The technical problem to be solved is to provide a cheaper rotor sensor target for
magnetic bearings which remedies the above-mentioned problems or drawbacks and in
particular exhibits good operational conditions and reduces the risks of damaging
the magnetic portion of a rotor axial or axial-radial sensor target, even if the sensor
target is subjected to a wide range of temperatures during operation.
[0012] In particular, the invention aims at improving the easiness of a manufacturing process,
enabling a lower cost and a high serial manufacturing process.
[0013] The invention is defined in the appended claims.
[0014] The invention more specifically relates to a rotor sensor target for magnetic bearings,
comprising a ring-shaped assembly of magnetic material mounted on a generally ring-shaped
assembly of non-magnetic material, which are coaxially arranged and mounted on a shaft
having a longitudinal axis of rotation X'-X, characterized in that said generally
ring-shaped assembly of non-magnetic material comprises at least one ring-shaped slit
having said longitudinal axis X'-X.
[0015] Preferably, the generally ring-shaped assembly of non-magnetic material is made of
a relatively cheap material such as aluminum.
[0016] According to a first embodiment, the generally ring-shaped assembly of non-magnetic
material comprises a set of first and second independent rings within which is interposed
the ring-shaped assembly of magnetic material.
[0017] According to a second embodiment, the generally ring-shaped assembly of non-magnetic
material comprises a first independent ring located on one side of the ring-shaped
assembly of magnetic material and a second ring located on the other side of the ring-shaped
assembly of magnetic material along the longitudinal axis X-X', the second ring being
a projection of a bigger ring extending internally beneath the first independent ring
and the ring-shaped assembly of magnetic material along the longitudinal axis X-X'.
[0018] The ring-shaped slit provides flexibility which permits operation over a wide range
of temperatures without risking of damaging the ring-shaped assembly of magnetic material.
[0019] Due to a specific shape of the sensor parts including at least one ring-shaped slit,
it is possible to use less resistant non-magnetic materials such as aluminum, instead
of e.g. Inconel or hi-grade brass for building a rotor sensor target for magnetic
bearings.
[0020] The improved shape of the sensor target permits the use of a material such as aluminum
which has sufficient strength to withstand high speeds and different temperatures
without losing contact with the main shaft and without breaking.
[0021] The present invention allows using the flexibility of the components of the target
to be able to keep all parts in place without over stressing the components, which
may thus be made of less resistant material such as aluminum which reduces the cost
of the sensor.
[0022] At high temperatures, notwithstanding the fact that the coefficient of thermal expansion
of the aluminum is higher than the coefficient of thermal expansion of iron laminations,
due to the specific configuration of the sensor target according to the invention,
at high temperatures the iron laminations will not be stressed over the yield tensile
strength and therefore the target will remain suitable for operation at lower temperatures,
contrary to the conventional structures proposed in the prior art.
[0023] Due to the provision of at least one ring-shaped slit, a ring under the iron laminations
or similar magnetic material may be made very thin and only supported at one or both
edges of the thinner ring making it more flexible and compensating the effort given
by the higher thermal expansion of a non-magnetic material such as aluminum. In this
way the stress on magnetic laminations will always be under the yield tensile strength.
It is therefore possible to work at higher temperatures even with a combination of
cheaper materials for the non-magnetic and magnetic materials such as aluminum and
iron laminations.
[0024] The invention may be implemented with various shapes and configurations for the assembly
of non-magnetic material.
[0025] According to a variant embodiment the at least one ring-shaped slit having the longitudinal
axis X'-X is located between a thicker ring-shaped body of non-magnetic material mounted
on the shaft and a thinner ring-shaped body of non-magnetic material mounted beneath
the ring-shaped assembly of magnetic material.
[0026] In such a case according to an advantageous embodiment the at least one ring-shaped
slit having the longitudinal axis X'-X is closed at both ends by narrow projections
of the thicker ring-shaped body of non-magnetic material mounted on the shaft.
[0027] According to another variant embodiment the at least one ring-shaped slit having
the longitudinal axis X'-X is provided within a thicker ring-shaped body of non-magnetic
material mounted on the shaft and located beneath the ring-shaped assembly of magnetic
material, the thicker ring-shaped body of non-magnetic material having a substantially
U-shape in longitudinal half cross-section along the longitudinal axis X-X'.
[0028] According to still another variant embodiment the at least one ring-shaped slit having
the longitudinal axis X'-X is provided within a ring-shaped body of non-magnetic material
mounted on the shaft and located beneath the ring-shaped assembly of magnetic material,
the ring-shaped body of non-magnetic material comprising a thicker ring-shaped portion
mounted on the shaft, a thinner ring-shaped portion located beneath the ring-shaped
assembly of magnetic material and a narrow portion bridging the thinner ring-shaped
portion and the thicker ring-shaped portion on one side of the at least one ring-shaped
slit, the ring-shaped body of non-magnetic material having a substantially C-shape
in longitudinal half cross-section along the longitudinal axis X-X'.
[0029] According to still another variant embodiment the sensor target comprises first and
second ring-shaped slits having the longitudinal axis X'-X which are provided within
a ring-shaped body of non-magnetic material mounted on the shaft and located beneath
the ring-shaped assembly of magnetic material, the ring-shaped body of non-magnetic
material comprising a first thin ring-shaped portion mounted on the shaft, a second
thin ring-shaped portion located between the first and second ring-shaped slits, a
third thin ring-shaped portion located beneath the ring-shaped assembly of magnetic
material, a first narrow portion bridging the first and second thin ring-shaped portions
on one side of the first ring-shaped slit, a second narrow portion bridging the second
and third thin ring-shaped portions on another side of the second ring-shaped slit,
the ring-shaped body of non-magnetic material having a substantially S-shape in longitudinal
half cross-section along the longitudinal axis X-X'.
[0030] The shaft may be made of carbon steel, whereas the magnetic material may be iron
laminations preferably silicon iron laminations.
[0031] The invention further relates to an axial-radial sensor for active magnetic bearings,
comprising a rotor sensor target as defined above.
[0032] Other characteristics and advantages of the invention appear from the following description
of particular embodiments, given as examples and with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033]
Fig. 1 is a longitudinal sectional view of components of a rotor sensor target according
to a first embodiment of the invention;
Fig. 2 is a longitudinal sectional view of components of a rotor sensor target according
to a second embodiment of the invention;
Fig. 3 is a longitudinal sectional view of components of a rotor sensor target according
to a variant of the first embodiment of the invention;
Fig. 4 is a longitudinal sectional view of components of a rotor sensor target according
to a third embodiment of the invention;
Fig. 5 is a longitudinal sectional view of components of a rotor sensor target according
to a variant of the third embodiment of the invention;
Fig. 6 is a longitudinal sectional view of components of a rotor sensor target according
to a fourth embodiment of the invention;
Fig. 7 is a longitudinal sectional view of components of a rotor sensor target according
to a variant of the second embodiment of the invention;
Fig. 8 is a longitudinal sectional view of components of a rotor sensor target according
to a variant of the fourth embodiment of the invention;
Fig. 9 is an axial half-section view of an example of a prior art active magnetic
bearing; and
Fig. 10 is a longitudinal sectional view of components of a rotor sensor target according
to an embodiment of the prior art.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] The present invention will be described in connection with preferred embodiments
which are given by way of examples.
[0035] The features of the different embodiments may be combined together unless otherwise
stated.
[0036] A typical arrangement of a first embodiment of the invention is illustrated in Fig.
1.
[0037] As shown in Fig. 1, in order to constitute a rotor sensor target on a shaft 20 having
a longitudinal axis X-X' and being made for example of carbon steel, a ring-shaped
assembly 30 of magnetic material, such as laminations of silicon iron, is coaxially
arranged with the shaft 20 and is mounted on a generally ring-shaped assembly of non-magnetic
material, such as aluminum, which is also coaxially arranged with the shaft 20 and
is mounted thereon and bonded thereto by any known means.
[0038] In the embodiment of Fig. 1, the assembly of non-magnetic material comprises a first
relatively thick ring-shaped body 60 which is directly bonded to the shaft 20 and
has two narrow slightly projecting parts or flanges 62, 63 on the outer surface of
this first relatively thick ring-shaped body 60.
[0039] The assembly of non-magnetic material further comprises a second relatively thin
ring-shaped part 50 which is fitted on the two narrow slightly projecting parts 62,
63 of the first relatively thick ring-shaped body 60, thus defining a ring-shaped
slit 61 between the first relatively thick ring-shaped body 60 and the second relatively
thin ring-shaped part 50. The second relatively thin ring-shaped part 50 is preferably
made of the same material as the first relatively thick ring-shaped body 60, such
as aluminum, but it is also possible to choose different materials. Finally, the ring-shaped
assembly 30 of magnetic material is interposed between first and second rings 41,
42 which are made of non-magnetic material, such as aluminum and define a set 40 of
rings bonded to the outer surface of the second relatively thin ring-shaped part 50.
[0040] The provision of a thin ring-shaped part 50 immediately under the ring-shaped assembly
of magnetic material 30 and the fact that such thin ring-shaped part 50 is only supported
at the two narrow slightly projecting parts 62, 63 constituting the edges of the first
relatively thick ring-shaped body 60, whereas a ring-shaped slit 61 is defined between
these edges 62, 63, provides flexibility to the thin ring-shaped part 50 and to the
whole sensor target. This avoids that the ring-shaped assembly of magnetic material
30 be subjected to undue stresses when the sensor is used in a wide range of temperatures.
[0041] The rotor sensor target according to the invention may be used in an axial or axial-radial
sensor comprising a conventional stator having a stack of ferromagnetic laminations
and electromagnet windings as shown e.g. in Fig. 9.
[0042] Fig. 2 illustrates a second embodiment which is generally similar to the embodiment
of Fig. 1 in as much as it comprises a ring-shaped assembly of magnetic material 30
interposed between first and second rings 41, 42 which are made of non-magnetic material,
such as aluminum and define a set 40 of rings bonded to an outer surface of another
ring-shaped part 70 made of non-magnetic material. In the embodiment of Fig. 2 the
elements which are identical to the elements of the embodiment of Fig. 1 bear the
same reference numerals and will not be described again in detail.
[0043] In the second embodiment of Fig. 2, one ring-shaped slit 71 is provided within a
thicker ring-shaped body 70 of non-magnetic material mounted on the shaft 20 and directly
located beneath the ring-shaped assembly of magnetic material 30. The thicker ring-shaped
body 70 of non-magnetic material has a substantially U-shape in longitudinal half
cross-section along the longitudinal axis X-X' and comprises first and second radial
flanges 72, 73 as well as a cylindrical portion 74 which is located directly beneath
the ring-shaped assembly of magnetic material 30. The cylindrical portion 74 may be
relatively thin and can be compared with the ring 50 of the embodiment of Fig. 1,
whereas the flanges 72, 73 may be compared to the edges 62, 63 of the thicker ring-shaped
body 60 of the
embodiment of Fig. 1.
[0044] Fig. 3 illustrates a variant embodiment of the sensor target of Fig. 1. The configuration
is very similar, but in the embodiment of Fig. 3, the second ring 42 of the embodiment
of Fig. 1 is replaced by a projection 51 of the relatively thin ring 50' which was
already present in the embodiment of Fig. 1.
[0045] Fig. 4 illustrates a third embodiment which is generally similar to the embodiment
of Fig. 1 in as much as it comprises a ring-shaped assembly of magnetic material 30
interposed between first and second rings 41, 42 which are made of non-magnetic material,
such as aluminum and define a set 40 of rings bonded to an outer surface of another
ring-shaped part 80 made of non-magnetic material. In the embodiment of Fig. 4 the
elements which are identical to the elements of the embodiment of Fig. 1 bear the
same reference numerals and will not be described again in detail.
[0046] In the third embodiment of Fig. 4, one ring-shaped slit 81 is provided within a thicker
ring-shaped body 80 of non-magnetic material mounted on the shaft 20 and directly
located beneath the ring-shaped assembly of magnetic material 30.
[0047] The ring-shaped slit 81 having a longitudinal axis X'-X is provided within a ring-shaped
body 80 of non-magnetic material mounted on the shaft 20 and located beneath the ring-shaped
assembly of magnetic material 30. The ring-shaped body 80 of non-magnetic material
comprises a thicker ring-shaped portion 82 mounted on the shaft 20, a thinner ring-shaped
portion 84 located beneath the ring-shaped assembly of magnetic material 30 and a
narrow portion 83 bridging the thinner ring-shaped portion 84 and the thicker ring-shaped
portion 82 on one side of the ring-shaped slit 81. The ring-shaped body 80 of non-magnetic
material thus has a substantially C-shape in longitudinal half cross-section along
the longitudinal axis X-X'. The thinner ring-shaped portion 84 and the ring-shaped
slit 81 of the third embodiment of Fig. 4 may be compared to the thin ring 50 and
the annular slit 61 of the first embodiment of Fig. 1 respectively.
[0048] Fig. 5 illustrates a variant embodiment of the sensor target of Fig. 4. The configuration
is very similar, but in the embodiment of Fig. 5, the second ring 42 of the embodiment
of Fig. 4 is replaced by a projection 85 of a thinner ring-shaped portion 84 of a
ring-shaped body 80' which was already present in the embodiment of Fig. 4.
[0049] Fig. 6 illustrates a fourth embodiment which is generally similar to the embodiment
of Fig. 1 in as much as it comprises a ring-shaped assembly of magnetic material 30
interposed between first and second rings 41, 42 which are made of non-magnetic material,
such as aluminum and define a set 40 of rings bonded to an outer surface of another
ring-shaped part 90 made of non-magnetic material. In the embodiment of Fig. 6 the
elements which are identical to the elements of the embodiment of Fig. 1 bear the
same reference numerals and will not be described again in detail.
[0050] In the third embodiment of Fig. 6, the sensor target comprises first and second ring-shaped
slits 92, 94 having a longitudinal axis X'-X which are provided within a ring-shaped
body 90 of non-magnetic material mounted on the shaft 20 and located beneath the ring-shaped
assembly of magnetic material 30. The ring-shaped body 90 of non-magnetic material
comprises a first thin ring-shaped portion 91 mounted on the shaft 20, a second thin
ring-shaped portion 93 located between the first and second ring-shaped slits 92,
94, a third thin ring-shaped portion 95 located beneath the ring-shaped assembly of
magnetic material 30, a first narrow portion 97 bridging the first and second thin
ring-shaped portions 91, 93 on one side of the first ring-shaped slit 92 and a second
narrow portion 96 bridging the second and third thin ring-shaped portions 93, 95 on
another side of the second ring-shaped slit 94. The ring-shaped body 90 of non-magnetic
material has a substantially S-shape in longitudinal half cross-section along the
longitudinal axis X-X'.
[0051] Fig. 7 illustrates a variant embodiment of the sensor target of Fig. 2. The configuration
is very similar, but in the embodiment of Fig. 7, the second ring 42 of the embodiment
of Fig. 2 is replaced by a projection 75 of the relatively thin cylindrical portion
74 of a ring-shaped body 70' which was already present in the embodiment of Fig. 2.
[0052] Fig. 8 illustrates a variant embodiment of the sensor target of Fig. 6. The configuration
is very similar, but in the embodiment of Fig. 8, the second ring 42 of the embodiment
of Fig. 6 is replaced by a projection 98 of a thinner ring-shaped portion 94 of a
ring-shaped body 90' which was already present in the embodiment of Fig. 6.
[0053] Generally speaking, the invention provides a simplification in the manufacturing
process, increases performance and reduces cost.
[0054] Although preferred embodiments have been shown and described, it should be understood
that any changes and modifications may be made therein without departing from the
scope of the invention as defined in the appended claims. Thus the features of the
different embodiments may be combined.
1. A rotor sensor target for magnetic bearings, comprising a ring-shaped assembly of
magnetic material (30) mounted on a generally ring-shaped assembly of non-magnetic
material (40, 50, 60; 40, 70; 50', 60; 40, 80; 41, 80'; 40, 90; 41, 90'; 41, 70'),
which are coaxially arranged and mounted on a shaft (20) having a longitudinal axis
of rotation X'-X, characterized in that said generally ring-shaped assembly of non-magnetic material (40, 50, 60; 40, 70;
50', 60; 40, 80; 41, 80'; 40, 90; 41, 90'; 41, 70') comprises at least one ring-shaped
slit (61; 71; 81; 92, 94) having said longitudinal axis X'-X.
2. The rotor sensor target according to claim 1, wherein said generally ring-shaped assembly
of non-magnetic material (40, 50, 60; 40, 70; 50', 60; 40, 80; 41, 80'; 40, 90; 41,
90'; 41, 70') is made of aluminum.
3. The rotor sensor target according to claim 1 or claim 2, wherein said generally ring-shaped
assembly of non-magnetic material (40, 50, 60; 40, 70; 40, 80; 40, 90), comprises
a set (40) of first and second independent rings (41, 42) within which is interposed
said ring-shaped assembly of magnetic material (30).
4. The rotor sensor target according to claim 1 or claim 2, wherein said generally ring-shaped
assembly of non-magnetic material (41, 50', 60; 41, 80'; 41, 90'; 41, 70'), comprises
a first independent ring (41) located on one side of said ring-shaped assembly of
magnetic material (30) and a second ring (51; 85; 98; 75) located on the other side
of said ring-shaped assembly of magnetic material (30) along said longitudinal axis
X-X', said second ring (51; 85; 98; 75) being a projection of a bigger ring (50';
80'; 90'; 70') extending internally beneath said first independent ring (41) and said
ring-shaped assembly of magnetic material (30) along said longitudinal axis X-X'.
5. The rotor sensor target according to any one of claims 1 to 4, wherein said at least
one ring-shaped slit (61) having said longitudinal axis X'-X is located between a
thicker ring-shaped body (60) of non-magnetic material mounted on said shaft (20)
and a thinner ring-shaped body (50; 50') of non-magnetic material mounted beneath
said ring-shaped assembly of magnetic material (30).
6. The rotor sensor target according to claim 5, wherein said at least one ring-shaped
slit (61) having said longitudinal axis X'-X is closed at both ends by narrow projections
(62, 63) of said thicker ring-shaped body (60) of non-magnetic material mounted on
said shaft (20).
7. The rotor sensor target according to any one of claims 1 to 4, wherein said at least
one ring-shaped slit (61) having said longitudinal axis X'-X is provided within a
thicker ring-shaped body (70; 70') of non-magnetic material mounted on said shaft
(20) and located beneath said ring-shaped assembly of magnetic material (30), said
thicker ring-shaped body (70; 70') of non-magnetic material having a substantially
U-shape in longitudinal half cross-section along said longitudinal axis X-X'.
8. The rotor sensor target according to any one of claims 1 to 4, wherein said at least
one ring-shaped slit (81) having said longitudinal axis X'-X is provided within a
ring-shaped body (80; 80') of non-magnetic material mounted on said shaft (20) and
located beneath said ring-shaped assembly of magnetic material (30), said ring-shaped
body (80; 80') of non-magnetic material comprising a thicker ring-shaped portion (82)
mounted on said shaft (20), a thinner ring-shaped portion (84) located beneath said
ring-shaped assembly of magnetic material (30) and a narrow portion (83) bridging
said thinner ring-shaped portion (84) and said thicker ring-shaped portion (82) on
one side of said at least one ring-shaped slit (81), said ring-shaped body (80; 80')
of non-magnetic material having a substantially C-shape in longitudinal half cross-section
along said longitudinal axis X-X'.
9. The rotor sensor target according to any one of claims 1 to 4, wherein it comprises
first and second ring-shaped slits (92, 94) having said longitudinal axis X'-X which
are provided within a ring-shaped body (90; 90') of non-magnetic material mounted
on said shaft (20) and located beneath said ring-shaped assembly of magnetic material
(30), said ring-shaped body (90; 90') of non-magnetic material comprising a first
thin ring-shaped portion (91) mounted on said shaft (20), a second thin ring-shaped
portion (93) located between said first and second ring-shaped slits (92, 94), a third
thin ring-shaped portion (95) located beneath said ring-shaped assembly of magnetic
material (30), a first narrow portion (97) bridging said first and second thin ring-shaped
portions (91, 93) on one side of said first ring-shaped slit (92), a second narrow
portion (96) bridging said second and third thin ring-shaped portions (93, 95) on
another side of said second ring-shaped slit (94), said ring-shaped body (90; 90')
of non-magnetic material having a substantially S-shape in longitudinal half cross-section
along said longitudinal axis X-X'.
10. The rotor sensor target according to any one of claims 1 to 9, wherein said magnetic
material is silicon iron.
11. The rotor sensor target according to any one of claims 1 to 10, wherein said shaft
(20) is made of carbon steel.
12. An axial-radial sensor for active magnetic bearings, comprising a rotor sensor target
according to any one of claims 1 to 11.